Mercruiser 485 overheating

[RD Durham]

I have a 1984 19’ Cobia with a Mercruiser 485 I4 engine. Last fall on my last trip to the lake the impeller came loose from the camshaft and chewed itself up. The aluminum pieces from the impeller took out the two seals on the front of the camshaft and all the coolant went into the engine block and I spun two bearings on the crankshaft. I have a parts engine that had all new parts (owner left it uncovered and it locked up from rain/snow getting into the engine cylinders). Between the two engines I got one engine rebuilt and running. My problem is that I decided that I had enough with the stock water pump. This is not the first time I’ve had issues with the water pump, though the prior problems were not as drastic. I bought an external coolant pump from Speed Masters (Speed Masters Water Pump) that can be used on V8 automobile engines. I fabricated hose connectors for each end and installed the water pump in place of my oil cooler. I relocated the oil cooler to the back of the engine, mounting it in the cross over hose that goes from the exhaust manifold to the intake manifold. All hoses still have the internal coil springs. I cut off the end of the camshaft and put two Welch (freeze) plugs in the timing chain cover where the camshaft had been.

I now have overheating issues. Last year I had no issues with overheating, except when the impeller in my outdrive went out. This time, initially when engine overheated the front hose going from the exhaust manifold to the heat exchanger would get to hot to hold, but not as hot as the engine. The back hose on the heat exchanger would stay warm and I could hold it without a problem. I found out that according to my shop manual, I had the wrong thermostat in the exhaust manifold and replaced it with the one shown in my shop manual (Pt#99155). It still overheated, but now both hoses on the heat exchanger stay colder than the engine (100 degrees compared to 200 degrees) while the engine goes up to 200 degrees before I can shut it off. The heat gauge goes up slowly until it hits approximately 160 degrees then it goes up fast. Thinking the orientation on the external coolant pump might be wrong, I flipped the coolant pump and still have the same problem with it overheating. I am using a “muff” in the outdrive connected to a garden hose to supply raw water to the outdrive water pump and heat exchanger. I am pretty sure I don’t have a blockage in the cooling system as I’m putting approximately 3 gallons of antifreeze in the engine and heat exchanger. I am removing the purge plug from the top of the exhaust manifold to be sure all the air gets out of the system when putting coolant back into the engine. Before I put antifreeze back into the engine/heat exchanger, I filled the engine with water and I pulled the hose from the heat exchange that went to the thermostat goose neck. I then I turned the external coolant pump on and I had water being pushed out of the end of the hose. So I’m pretty sure the pump is working and can pump water through the engine and the heat exchanger.

 

[gonzo]

If you only have water in the block, you can run the engine with the hose circulating water through the block and make sure it doesn't overheat. If it doesn't, there is either a problem with the heat exchanger of the water circulation through it. If the pump is of the type to replace an automotive circulation pump, it probably doesn't have enough pressure.

 

[RD Durham]

The engine uses the larger 4" heat exchanger so only antifreeze in the block. The heat exchanger seems to be clear of any obstructions. I've blown air through it and it doesn't' seem to have any blockage. I really don't want to go back to the stock water pump set up unless I have to on this boat. I've seen on some other forums, especially street rods, where they've used this engine with an external coolant pump but no ones seems to put up an specifications for the pump on how many gallons an hour or minute it flows with the stock coolant system. What specifications should I look for in an external coolant pump?

Checking some other websites out that talk about the Mercruiser 470/485 overheating it does seem as you may be correct that the flow rate is not sufficent. In one discussion a owner posted that on his engine with the stock impeller water when he removed the heater exchanger hose water would shoot out of hose like from a garden hose. When I removed the heater exchanger hose water flowed out but not all that fast, certainly not like a garden hose turned to full.

I've been looking at other external coolant pumps in case this one is not up to the task. One is a pump sold by Parts For Speed Pt#PRP6055 [Remote Coolant Pump PRP6055 - eBay flows 25 gpm. I've also looked at a Meziere electric coolant pump #WP136S that flows 20 gpm. I checked with Speed Masters about my current external coolant pump it should be flowing 35 gpm.

 

[gonzo]

You can blow air through a 1/4" hole and it will flow freely. Water will need less restriction.

 

[PAR]

Boil the exchanger . . .

 

[RD Durham]

Thanks, I may do that. Still looking at it but it might be next spring. It ran fine last year with the stock set up, until the impeller came off the camshaft. If worse comes to worse I have the parts to go back to stock, but I'd rather not if I can get this to work.